Research On The Control Techniques Of High-precision And Flexibility For CNC Hydraulic Press Brake

Press brake is the specialized equipment for sheet bending. Due to the operation simplicity and process generality, it is widely applied in sheet metal industry. Nowadays, subject to the costs of high-power AC servo motor and its driver, the press brake based on servo motor driver directly is only applied to the low tonnage. The CNC hydraulic press brake is the mainstream structural form of press brake in domestic industry. With the increasing demands for product precision and equipment automation, the research on the control techniques of high-precision and flexibility for CNC hydraulic press brake is of important practical significance. In this paper, the contents of research on the above questions includes high-precision dual-axis positioning algorithm, flexible auxiliary system control, specialized CNC system and angle full closed-loop control etc.Nowadays, the electro-hydraulic servo synchronous system and full closed-loop positioning control is widely used as the synchronous system and positioning mode of CNC hydraulic press brake. Because of the nonlinear, time-delay and time-varying characteristics of the electro-hydraulic servo synchronous system, high-precision dual-axis positioning algorithm is a key issue for product precision, and is also one of the most core and basic functions. In this paper, transfer function model of valve controlled asymmetric hydraulic cylinder system is established, and then a novel high-precision dual-axis positioning algorithm is presented on this basis. Feature points and parameters of Y axes are interpolated into the velocity and displacement arrays of points based discrete S-curve acceleration and deceleration. In five different bending working steps, a piecewise PID and velocity feedforward algorithm is proposed, and specifically displacement-time curve is switched to velocity-displacement curve for ensuring high-precision positioning in pressing working step at different lower dead point (LDP). A cross-coupling control method of dual-axis is presented to obtain high synchronous precision. The experimental results show that the proposed algorithm is satisfactory robustness and can meet the requirements of sheet bending.For CNC hydraulic press brake with "3+1" standard configuration, the computing models of bending force, back gauging positioning and deflection compensation are established, then specific control methods are proposed in this paper. In addition, individual requirements of different press brake manufacturers set great demands on flexibility of CNC system, and the configurations are as simple as possible. A method to ehance flexibility of CNC system with soft-PLC is proposed in this paper.In the sheet metal processing workshop, along with shearing machine and laser cutting machine the press brake is one link in the’cutting-punching-bending’production line. Thus the working environment is relatively poor. A stable numerical control system is needed. In this paper, open sheet bending control system is established based on PC. This hardware platform of the system is the Huazhong type8system developed by Huazhong numerical control Co., Ltd (HNC). It is designed with module method with high extendibility. The software system is run on Windows NT platform. The user interface and process planning modules are developed with C++. And the core controlling algorithm is based on Windows Driver Model (WDM). In practice, this system also shows excellent features such as operation simplicity, automation reliability and high extendibility.Springback is a common phenomenon in sheet bending processing. It greatly affects the precision of the products. At present, the bending angle control mostly applies the half closed loop method, which punch displacement is the adjustment parameter for bending angle controlling. While the bending angle is correlated with a lot of factors, numerical control system can only manipulate the punch displacement other than the errors caused other factors. A non-contact measurement method is proposed Based on laser angle online measurement independently developed. According to measured angle after springback, displacement needed for the objective angle can be calculated. Then the programmed value of punch displacement is adjusted. The required bending angle can be achieved with only one trial bending.The research outcome is applied in the HNC-SPB60press brake NC system, cooperating with HNC. And this system has already been implemented on the press brakes developed by Hubei tri-ring metalforming equipment Co., Ltd. and Jiangsu Jinfangyuan CNC Machine Co., Ltd. It is now in the commercial promotion stage.